Process of treating a boron nitride crucible with molten aluminum



Nov. 13, 1962 c. A. BAER ETAL PROCESS OF TREATING A BORON NITRIDE 3063865 CRUCIBLE WITH MOLTEN ALUMINUM Filed June 5, 1957 O XXXXXX Fig2 AL AND EN DARK BROWN FILM United States Patent OfiFice 3,063,865 PROCESS OF TREATING A BORON NTTRIDE CRUCIBLE WITH MOLTEN ALUMINUM Charles A. Baer, Needham, Philip J. Clough, Reading, and Robert W. Steeves, Nahant, Mass., assignors, by mesne assignments, to National Steel Corporation, a corporation of Delaware Filed June 3, 1957, Ser. No. 662,991 9 Claims. (Cl. 117-66) This invention rel-ates to coating and more particularly to coating of a substrate by the vapor deposition of aluminum under a high vacuum.

A principal object of the present invention is to provide an improved source of aluminum vapors having a long source life at elevated aluminum temperatures on the order of 1200 C. to 1350 C.

Another object of the invention is to provide an improved method of handling such an aluminum vapor source so as to provide minimum transfer of radiant energy to the substrate and to furnish optimum source life.

Other objects of the invention will in part be obvious and will in part appear hereinafte The invention accordingly comprises the process involving the several steps and the relation and the order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein:

FIG. 1 is a diagrammatic, fractional sectional view of one preferred form of the invention; and

FIG. 2 is a diagrammatic, fractional sectional view of another preferred form of the invention.

In the vapor deposition of aluminum onto a substrate in a vacuum chamber, one of the principal problems of the prior art has been that of providing a suitable source for the aluminum vapors. We have discovered that a crucible of boron nitride, particularly when treated to provide a surface that is readily wet by molten aluminum, has several outstanding advantages as a support for holding molten aluminum while it is heated to evaporation temperatures on the order of 1200 C. to 1350 C. The crucible seems to be almost completely inert to attack by molten aluminum even at very high temperatures such as 1300 C. The crucible can be made so that it is readily wet by molten aluminum, whereby all of the hot areas of the crucible are covered by molten aluminum, this aspect of the invention being particularly important when the crucible is utilized for coating heat-sensitive substrates such as plastics or paper.

While boron nitride is theoretically an excellent insulator, even at elevated temperatures, it has been found that, when the surface has been treated so as tobe wet by molten aluminum, the whole crucible seems to be conductive. Accordingly, it makes an excellent induction-heated source of aluminum vapors. it has been found that such a crucible, even when it has been evaporated to dryness, continues to maintain sufiicient conductivity to permit its being kept at an elevated temperature by induced currents. The fact that the crucible is conducive permits a large mass of molten aluminum to be maintained in the crucible even though some induction currents are created in the molten aluminum. In a nonconducting crucible it is impossible to maintain a full charge in a crucible since the induction field will literally throw the molten aluminum out of the crucible. With a boron nitride crucible, wetted by molten aluminum, there Patented Nov. 13, 1952 is sufi'icient coupling to the crucible so that a completely full crucible may be maintained.

In one preferred embodiment of the invention, the crucible is formed of pressed boron nitride, the crucible having 3 inches O.D., 2% inches 1.1)., and being 2 /2 inches high. The crucible as formed was white, had a somewhat greasy feel and could be easily scratched with a thumb nail. This crucible was placed inside of an induction coil, fed by a 9600 c.p.s. induction source, a layer of aluminum silicate refractory being placed between the induction coil and the outside of the boron nitride crucible. The crucible and coil assembly were mounted in a vacuum chamber which was then pumped down to a pressure of less than 1 micron Hg abs. A solid charge of aluminum was placed in the crucible and the power was turned on. During the initial heating of the aluminum, there was no coupling to the crucible, and only the aluminum charge became heated. As its temperature was raised, it became molten and it formed an up-right mass which was confined by the electromagnetic field. The aluminum was heated to a high temperature on the order of 1300 C. and gradually heated the crucible by radiation. During this heating of the crucible, very small portions of the crucible surface spalled off. The reason for this spalling during the initial heating of the crucible is not completely understood. It is believed that some of these pieces fell into the molten aluminum and may have been dissolved thereby. The aluminum soon started to wet the bottom inner surface of the crucible where it was in contact with the surface and slowly crept up the inner surfaces of the crucible Wall until it had wet the whole inner and top surfaces of the crucible. As the aluminum wet the crucible, the crucible became conductive and became heated to the same temperature (eg 1300 C.) as the molten aluminum. At this point the molten aluminum was only slightly affected by the electromagnetic field and now filled the crucible entirely, running up all inner surfaces of the crucible and covering the top surface as Well. Accordingly, all of the heated portions of the crucible were covered by high temperature molten aluminum. This wetting of the crucible took somewhat more than one-half hour.

Thereafter the crucible was utilized to coat a moving substrate. The crucible evaporated on the order of 3.5 grams per minute at a steady rate. After several hours, during which time solid aluminum had been periodically fed to replenish the supply of aluminum in the crucible, the power was turned off. The vacuum tank was opened while the aluminum in the crucible was still molten and the molten aluminum was dumped out of the crucible to prevent its freezing Within the crucible with the danger of cracking the crucible.

The crucible was examined carefully and it was found that, aside from the few instances of spalling which had been noticed in the earlier parts of the heat-up, the crucible was essentially free of erosion or corrosion. Its surface was covered with a thin layer of aluminum and aluminum oxide which had frozen thereon. 1

The crucible was replaced in the induction coil and a charge of solid aluminum was put in the crucible and the tank was re-evacuated. Turning on the power again, the crucible promptly heated up along with the aluminum charge and 'in about 12 minutes the whole mass of aluminum and the crucible were up to a temperature of about 1300 C. The aluminum wet all of the visible crucible surface. The crucible was tested for several hours, periodic additions of aluminum being made during this time. The crucible was evaporated to dryness and removed. When examined, after cooling, the crucible showed a rather metallic surface which appeared to be an extremely thin aluminum film or a reaction product between aluminum and the boron nitride of the crucible.

' melted therein. After 20 minutes or so, it was completely wet by molten aluminum again at elevated temperatures. I

The crucible was checked to ascertain the condition of the Surface under the aluminum film which remained after the bulk of the aluminum had been poured out of the crucible. At this point the crucible had the structure shown in FIG. 1. This check was made by immersing the crucible in a caustic solution (25% NaOH) for about 16 hours. At the end of this time there were no visible signs of metallic aluminum present on the surface of the crucible. Instead, the surface of the crucible was covered with a dark brown film (as indicated in FIG. 2) which could be scraped off the crucible with difiiculty.

Since certain changes may be made in the above proc- "ess and product without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

'1. In a process for coating a substrate with aluminum by vacuum vapor deposition techniques, the improvement which comprises forming a conducting surface on a crucible consisting essentially of boron nitride, melting aluminum in the boron nitride crucible by means of induced electrical currents from an induction coil surrounding the crucible, the conducting surface being suffi ciently thick to permit initial flow of induced current in intimate contact with the crucible to heat the crucible and to prevent undue stirring of molten aluminum in the crucible.

2. In the process of evaporating aluminum in an' evacuated chamber and depositing the evaporated aluminum on a moving substrate to form an aluminum coating on the substrate,.the improvement which comprises supporting the molten aluminum on a surface consisting essentially of the reaction product of aluminum with boron nitride while the aluminum is being heated to evaporation temperature.

3. In a process for the deposition of analuminum film on a substrate by thermal evaporation of molten aluminum in a vacuum wherein the aluminum is heated to a temperature sufficiently above its melting point to evaporate the aluminum and an aluminum coating is condensed on a moving substrate, the improvement which comprises supporting the molten aluminum, while the aluminum is heated to its evaporation temperature, in a crucible having a surface consisting essentially of the product of the reaction between aluminum and boron nitride.

4. In a process for the deposition of an aluminum film on a substrate by thermal evaporation of molten aluminum in a vacuum wherein the aluminum is heated to a temperature sufficiently above its melting point to evaporate the aluminum and an aluminum coating is condensed on a moving substrate, the improvement which comprises supporting the molten aluminum, while the aluminum is heated to its evaporation temperature, in a crucible having a surface consisting essentially of the product resulting from reaction between molten aluminum and boron nitride.

5. in a process for the deposition of an aluminum film on a substrate by thermal evaporation of molten aluminum in a vacuum wherein the aluminum is heated to a temperature sufiicien-tly above its melting point to evaporate the aluminum and an aluminum coating is condensed on a moving substrate, the improvement which comprises supporting the molten aluminum, while the aluminum is heated to its evaporation temperature, in a crucible of boron nitride which has been made conductive by reaction with molten aluminum at an elevated temperature on the order of 1100 C. and above.

6. In a process for the deposition of an aluminum film on a substrate by thermal evaporation of molten aluminum in a vacuum wherein the aluminum is heated to a temperature sufliciently above its melting point to evaporate the aluminum and an aluminum coating is condensed on a moving substrate, the improvement which comprises supporting the molten aluminum, while the aluminum is heated to its evaporation temperature, in a crucible having at least an inner stratum of the product of the reaction between aluminum and boron nitride in .position to contact the molten aluminum during evaporation.

7. A boron nitride crucible capable of being induc tively heated to red heat by an electromagnetic field of 9600 cycles per second, the crucible having an inner surface which has been wet by molten aluminum.

8. In a process for the deposition of an aluminum film on a substrate by thermal evaporation of molten aluminum in a vacuum wherein the aluminum is heated to a temperature sufficiently above its melting point to evaporate the aluminum and an aluminum coating is condensed on a moving substrate, the improvement which comprises supporting the molten aluminum, while the aluminum is heated to its evaporation temperature, in a crucible having at least an inner stratum of the product of the reaction between aluminum and boron nitride in position to contact the molten aluminum during evaporation, the aluminum being heated to a temperature on the order of 1200 C. and above, and substantially completely emptying the crucible of its charge of molten aluminum prior to cooling the crucible to a temperature below the freezing point of aluminum.

9. The process of claim 8 wherein the aluminum is emptied by evaporating the crucible to dryness.

References Cited in the file of this patent UNITED STATES PATENTS 1,063,483 Weintraub June 3, 1913 1,570,802 Bichowsky I an. 26, 1926 2,201,049 Moore May 14, 1940 2,726,160 Ueltz Dec. 6, 1955 2,772,318 Holland Nov. 27, 1956 FOREIGN PATENTS 73 1,990 Great Britain June 15, 1955 OTHER REFERENCES Vapor-Plating, Powell et al., John Wiley and Sons, 1955, pages 101, 111, 112, and 117 relied on. 

8. IN A PROCESS FOR THE DEPOSITION OF AN ALUMINUM FILM ON A SUBSTRATE BY THERMAL EVAPORATION OF MOLTEN ALUMINUM IN A VACUUM WHEREIN THE ALUMINUM IS HEATED TO A TEMPERATURE SUFFICIENTLY ABOVE ITS MELTING POINT TO EVAPORATE THE ALUMINUM AND AN ALUMINUM COATING IS CONDENSED ON A MOVING SUBSTRATE, THE IMPROVEMENT WHICH COMPRISES SUPPORTING THE MOLTEN ALUMINUM, WHILE THE ALUMINUM IS HEATED TO ITS EVAPORATION TEMPERATURE, IN A CRUCIBLE HAVING AT LEAST AN INNER STRATUM OF THE PRODUCT OF THE REACTION BETWEEN ALUMINUM AND BORON NITRIDE IN POSITION TO CONTACT THE MOLTEN ALUMINUM DURING EVAPORATION, THE ALUMINUM BEING HEATED TO A TEMPERATURE ON THE ORDER OF 1200*C. AND ABOVE, AND SUBSTANTIALLY COMPLETELY EMPTYING THE CRUCIBLE OF ITS CHARGE OF MOLTEN ALUMINUM PRIOR TO COOLING THE CRUCIBLE TO A TEMPERATURE BELOW THE FREEZING POINT OF ALUMINUM. 